The contrast in magnetic images is determined by differences in nuclear spin relaxation rates. The administration of agents that may selectively control magnetic relaxation rates provides control of the information content of a magnetic image. 'This proposal addresses the fundamental physical and chemical aspects of controlling longitudinal or spin-lattice relaxation and transverse or spin-spin relaxation rates using a variety of paramagnetic and diamagnetic agents. Recent work has shown that the standard approaches to understanding magnetic relaxation are incomplete. The major difficulty is to understand how to control electron spin relaxation rates that limit the efficiency of nuclear spin relaxation induced by paramagnetic agents. The work proposed attempts to address these difficulties using model systems that are possible to control. In addition study of paramagnetic relaxation in systems where the relaxation center is immobilized on a protein matrix or in an aluminosilicate particle is proposed because these systems may be very efficient relaxation agents. The specific aims of the proposed work include definition of the conditions for optimal relaxation efficiency of the aluminosilicates; determination of the effectiveness of controlling the local micro-dynamic viscosity at paramagnetic centers as a means of increasing relaxation efficiency; measurement of metal complex ion rotational correlation times as a means of defining the role of the electron spin relaxation times in controlling the electron-nuclear coupling; measurement of electron spin relaxation rates in the context of determining whether controlling the internal stiffness as well as the symmetry of a metal complex can produce very long electron spin relaxation times; and, characterization of immobilized protein paramagnetic systems as highly efficient relaxation agents.